Three widely practiced techniques are presently in extensive commercial use in nondestructive object testing, namely, leakage flux detection, eddy current measurement and acoustic wave examination. The first two named techniques have in common transferring to the test object a magnetic flux either for eddy current generation in the surface thereof or for so saturating the object as to force flux exterior to the object where flaws exist. Present commercial versions of the acoustic wave technique are based upon the use of a magnetic field cooperatively with an acoustic wave generated by a piezoelectric device and applied to the object by a fluid medium couplant. The leakage flux and eddy current techniques may be categorized as magnetic and the magnetic/acoustic technique may be categorized as ultrasonic.
In the ultrasonic area, a development of the recent decades offers benefit in its elimination of the couplant medium. The couplant has been a bothersome element and occasions have arisen where, despite that the test environment or test object has called for ultrasonic practice as uniquely applicable, the testing has either been done by other technique or has been foregone since use of the couplant is not permitted. The referenced recent development, as set forth, for example, in U.S. Pat. Nos. 3,460,063 and 3,583,213, is the observation of the phenomenon of electromagnetically inducing an acoustic wave in an object under steady-state magnetic field influence. While the theoretical aspects of this technique are extensively treated in published literature, the development has not shown industry-wide impact to this time.
One characteristic of known nondestructive testing apparatus and methods is that the various systems are made commercially available independently of one another. Thus, present commercial magnetic category products are for either leakage flux testing or eddy current measurement to detect object characteristics and do not incorporate ultrasonic apparatus of the piezoelectric type or of EMAT (electromagnetic acoustic transducer) type, and vice versa. The user industry has not heretofore been provided with apparatus and methods which look to the testing of objects commonly by both magnetic and ultrasonic techniques, particularly the more conveniently usable of the known ultrasonic techniques.